And here in suburban San Diego, the fears coincided with a new opportunity for precautionary action.

In June, the County Sheriff's department launched an online map of sex offenders, allowing parents to plug in addresses or zip codes and view sprinklings of colored dots, each dot representing the home of a convicted sex offender.

Logging into the website, a group of suburban mothers here was shocked to see one of those dots perched atop the home of a respected father and friend. That problem, they thought, was for other places. Not University City, an upscale enclave where neat houses nestle along winding roads and the sounds of children playing float through cul-de-sacs on warm autumn evenings.

Hoping to allay their concerns, Marie  who asked that her last name not be published  and two neighbors raced downtown to police headquarters, where they could view details behind the dots: photos, names, and charges.

To their horror, the familiar face appeared onscreen  and sparked an awareness that transformed the emotional landscape of this seemingly imperturbable place.

The discovery that their neighbor had been a sex offender slices to a core dilemma of the electronic age: the balance between the right to privacy and the right to know. Designed to protect children, the database can also spur panic and spark a rabid curiosity that feels eerily like a witch hunt to neighbors and convicts alike.

Megan's Law and questions of justice

While the online map simply delivers public information, its use raises difficult questions.

Margo Schlanger, an assistant professor at Harvard Law School and a former fellow at Harvard's Center for Ethics and the Professions, suggests that the map itself is not the issue. The real dilemma is broader: "Is this a good way to arrange a social system, that there is always this stigma?" she asks. "You have to ask yourself if this permanent scarlet letter is justified."

The online map and database are a result of Megan's Law, named after Megan Kanka, a New Jersey girl raped and killed by a child molester who lived across the street.

After Marie learned about her neighbor, her quandary was what to do. "I wanted to make sure I wasn't engaging in malicious gossip," she says.

Though she recoiled from contact with the man's wife, she had no legal recourse: Once offenders are off parole, they're free to live anywhere. Still, Marie became obsessed with the map, typing in specific addresses to test her suspicions of other neighbors.

"I found a young guy down the street from us is also [a sex offender]," Marie says. "If my daughter ever starts walking to school, it won't be in that direction."

In downtown San Diego, the map shows 367 dots; in upscale Point Loma, 22; in quiet La Jolla, 11.

Yet for Nancy, the wife of the sex offender Marie found on the map, a sense of injustice is palpable. After her husband's past was made public, she says, the couple lost all their friends. Everyone, it seemed, was talking about them: One neighbor learned of the situation after overhearing others in the supermarket.

Nancy's problem with the online map and database is its conflation of violent and nonviolent offenders and a lack of detail. "Every sex offender is not David Westerfield," she says, referring to the San Diego man recently convicted of kidnapping and murdering seven-year-old Danielle van Dam. According to Nancy, her husband's offense, which involved a boy fondling him, occurred 12 years ago. "At some point," Nancy says, "I hope society will say this person has been punished for his crime; let's give him another chance."

A dilemma of fear and protection

Parents routinely panic after checking out the map, says Jim Ryan, a detective in the Sex Offenders Registration Unit at the San Diego Police Department.

First, the numbers are shocking. And then, says Ryan, "People are always surprised that an offender can look just like one of them and live right beside them."

But Detective Ryan says that easy access to such information is intended to protect children, and not make sex offenders into pariahs. "It's important for parents to know who is in their backyard," he says.

Michael Nuccitelli, a New York psychologist who has worked with sex offenders, agrees. "The nature of pedophilia is that the pedophile can never be cured," he says. Although treatment can help arrest the behavior, the desire will persist, he says, and recidivism rates are high.

Among the mothers who rushed to the police station, this was the summer's major topic of discussion. None will allow their children near Nancy's home, though playdates there were commonplace last year. One mother sent her son to a different school this fall. And the principal of the neighborhood elementary school told Nancy's husband that a new state law prohibits him from volunteering there.

As autumn rolls in, the map remains a divining rod for worried parents. Before the map was online, and before the van Dam murder, few parents came down to headquarters to view the sex-offenders database.